1. Field of Invention
This invention pertains to a method and an apparatus that forms ablated features in substrates such as by laser ablation of polymer substrates for inkjet print head applications.
2. Description of the Related Art
The laser ablation of features on polymer materials using a mask and imaging lens system is well known. In this process, features on the mask are illuminated with laser light. The laser light that passes through the transparent features of the mask is then imaged onto the substrate such as a polymeric film where the ablation process occurs.
FIG. 1 illustrates a basic layout of a conventional excimer laser machining system 10. Typically, the system 10 is controlled by a computer 12 with an interface to the operator of the system. The computer 12 controls the firing of the pulsed laser system 24 and a low speed, low resolution servo system 14. The function of the servo system 14 is to position the mask 16 and substrate chuck 18 for proper registration of the laser milled pattern with respect to other features on the substrate 19 prior to ablation of substrate 19. For this purpose, a vision system (not shown) is often interfaced to the computer system. The servo system 14 or computer 12 may control an attenuator module 20, to vary the amount of UV radiation entering the system. Alternatively, the laser pulse energy may be varied by adjusting the laser high voltage or a control set point for energy, maintained by the laser's internal pulse energy control loop.
The UV beam path is indicated in this figure with arrows 22(not intended to be actual ray paths, which are not typically parallel) which show the flow of UV energy within the system. The UV power originates at the pulsed excimer laser 24. The laser 24 typically fires at 100-300 Hz for economical machining with pulses that have a duration of about 20-40 nanoseconds each. The typical industrial excimer laser is 100-150 watts of time average power, but peak powers may reach megawatts due to the short duration of the pulse. These high peak powers are important in machining many materials.
From the output end of the laser, the UV energy typically traverses attenuator 20; however, this is an optional component not present in all laser machining systems. The attenuator 20 performs either or both of two possible functions. In the first function, the attenuator 20 compensates for the degradation of the optical train. The attenuator 20 thus used, allows the laser to run in a narrow band of pulse energies (and hence a restricted range of high voltage levels), allowing for more stable operation over the long term. With new optics in the system, the attenuator 20 is set to dissipate some of the power of the laser. As the optics degrade and begin to absorb energy themselves, the attenuator 20 is adjusted to provide additional light energy. For this function, a simple manual attenuator plate or plates can be used. The attenuator plates are typically quartz or fused silica plates with special dielectric coatings on them to redirect some of the laser energy toward an absorbing beam dump within the attenuator housing.
The other possible function of the attenuator 20 is for short term control of laser power. In this case, the attenuator 20 is motorized with either stepper motors or servo system, and the attenuator is adjusted to provide the correct fluence (energy per unit area) at the substrate for proper process control.
From the attenuator 20, the UV energy propagates to a beam expansion telescope 26 (optional). The beam expansion telescope 26 serves the function of adjusting the cross sectional area of the beam to properly fill the entrance to the beam homogenizer 28. This has an important effect on the overall system resolution by creating the correct numerical aperture of illumination upon exit from the homogenizer. Typical excimer laser beams are not symmetric in horizontal vs. vertical directions. Typically, the excimer beam is described as "top hat-gaussian," meaning that between the laser discharge direction (usually vertical), the beam profile is "top hat" (initially relatively flat and dropping off sharply at the edges). In the transverse direction, the beam has a typical intensity profile that looks qualitatively gaussian, like a normal probability curve.
The expansion telescope 26 allows some level of relative adjustment in the distribution of power in these directions, which reduces (but does not completely eliminate) distortion of the pattern being imaged onto the substrate 19 due to the resolution differences in these two axes.
Between the expansion telescope 26 and homogenizer 28 is shown a flat beam folding mirror 30. Most systems, due to space limitations, will contain a few such mirrors 30 to fold the system into the available space. Generally, the mirrors may be placed between components, but in some areas, the energy density or fluence can be quite high. Therefore, mirror locations are carefully chosen to avoid such areas of high energy density. In general, the designer of such a system will try to limit the number of folding mirrors 30 in order to minimize optics replacement cost and alignment difficulty.
The UV light next enters the beam homogenizer 28. The purpose of the homogenizer 28 is to create a uniformly intense illumination field at the mask plane. It also determines the numerical aperture of the illumination field (the sine of the half angle of the cone of light impinging on the mask), which as stated above, has an impact on overall system resolution. Since certain parts of the excimer beam are hotter than others, uniform illumination requires that the beam be parsed into smaller segments which are stretched and overlaid at the mask plane. Several methods for this are known in the art, with some methods being based on traditional refractive optics, e.g., as disclosed in U.S. Pat. Nos. 4,733,944 and 5,414,559, both of which are incorporated herein by reference. The method may also be based on diffractive or holographic optics, as in U.S. Pat. No. 5,610,733, both of which patents are incorporated by reference, or on continuous relief microlens arrays (described in "Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing", Nikoladjeff, et. al, Applied Optics, Vol 36, No. 32, pp. 8481-8489, 1997).
From the beam homogenizer 28 the light propagates to a field lens 32, which serves to collect the light from the homogenizer 28 and properly couple it into the imaging lens 34. The field lenses 32 may be simple spherical lenses, cylindrical lenses, anamorphic or a combination thereof, depending on the application. Careful design and placement of field lenses 32 are important in achieving telecentric imaging on the substrate side of the lens 32.
The mask 16 is typically placed in close proximity to the field lens 32. The mask 16 carries a pattern that is to be replicated on the substrate 19. The pattern is typically larger (2 to 5 times) than the size of the pattern desired on the substrate 19. The imaging lens 34 is designed to de-magnify the mask 16 in the course of imaging it onto the substrate 19. This has the desired property of keeping the UV energy density low at the mask plane and high at the substrate plane. High de-magnification usually imposes a limit on the field size available at the substrate plane.
The mask 16 may be formed from chromium or aluminum coated on a quartz or fused silica substrate with the pattern being etched into the metallic layer by photolithography or other known means. Alternatively, the reflecting and/or absorbing layer on the fused silica mask substrate 16 may comprise a sequence of dielectrics layers, such as those disclosed in U.S. Pat. Nos. 4,923,772 and 5,298,351, both of which are incorporated herein by reference.
The purpose of the imaging lens 34 is to de-magnify and relay the mask pattern onto the substrate 19. If the pattern is reduced by a factor of M in each dimension, then the energy density is raised by M.sup.2 multiplied by the transmission factor of the imaging lens 34 (typically 80% or so). In its simplest form, the imaging lens 34 is a single element lens. Typically, the imaging lens 34 is a complex multi-element lens designed to reduce various aberration and distortions in the image. The imaging lens 34 is preferably designed with the fewest elements necessary to accomplish the desired image quality in order to increase the optical throughput and to decrease the cost of the imaging lens 34. Typically, the imaging lens 34 is one of the most expensive parts of the beam train.
As noted above, the imaging lens 34 creates a de-magnified image of the pattern of the mask 16 on the substrate 19. Each time the laser fires, an intense patterned area is illuminated on the substrate 19. As a result, etching of the substrate material results at the illuminated areas. Many substrate materials may be so imaged, especially polymeric materials. Polyimides available under various trade names such as Kapton.TM. and Upilex.TM. are the most common for microelectronic applications and inkjet applications.
The system 10 described in FIG. 1 is a "typical" system. For non-demanding applications, the system can be further simplified and still produce ablated parts, but with some sacrifice in feature tolerances, repeatability, or both. It is not unusual for systems to make some departure from this typical architecture, driven by the particular needs of the application.
There are many applications for laser ablation of polymeric materials. Some applications or portions thereof are not demanding in terms of tolerances, e.g., electrical vias, and the emphasis is on small size, high density features and low cost. Other applications require very demanding tolerances and repeatability. Examples of the latter applications are fluid flow applications such as inkjet print head nozzle manufacture and manufacture of drug dispensing nozzles. In these demanding applications, the requirements for exact size, shape, and repeatability of manufacture are much more stringent than the simpler conductive path features provided by a microelectronic via. The detailed architecture of the system is critical to obtaining tight tolerances and product repeatability. In addition, process parameters and the optical components all play important roles in obtaining the tightest possible tolerances, down to the sub-micron level.
As mentioned above, the invention relates to the formation of nozzles for inkjet print head applications and other fluid flow applications. During the firing of a thermal inkjet print head, a small volume of ink is vaporized. The vaporized ink causes a droplet of ink to shoot through an orifice (i.e., the nozzle) which is directed at the print media. The quality of thermal inkjet printing is dependent upon the characteristics of the orifice. Critical attributes of the orifice include the shape and surface condition of the bore.
One important aspect for fluid flow applications is the slope of the via walls. Vias made in the conventional manner have very steep wall slopes, with the slope dependent upon the incident radiation fluence (energy per unit area), and to a lesser extent, the number of laser pulses used to create the feature. Using conventional methods, very little can effectively be done to control or shape the via wall slope. One method is controlling the energy distribution of the radiation hitting the substrate. In a projection imaging system, this can be accomplished by placing ring shaped apertures on the mask, such as described in U.S. Pat. No. 5,378,137. However, the mask features used to create the hole profiles must be very small (sub-resolution for the imaging system), or they may be imaged into the ablated hole or via. The disadvantage of this method is that the small mask features can easily be damaged and also add difficulty and expense to the mask making process.
In a typical inkjet print head made currently in the industry, small ablated orifices or vias are made in the polymer film substrate at a concentration of about 300 or more ablated orifices per inch. The size of the orifices may vary depending upon the particular application, but generally have an exit diameter less than about 35 microns. The entrance orifice diameter is typically less than 100 microns, with an average entrance diameter of about 50 microns to about 60 microns being more typical. The objective of the invention described herein is to provide additional control over the shape of the orifice in addition to the traditional process controls of mask features, fluence, laser shots, and so forth in controlling the detailed shape of the orifice.
In addition to the ring-mask method described above, another method of shaping the orifice wall angle is to displace the beam using an optical method. This can be accomplished, for example, by spinning a flat or wedge-shaped optical element between the mask and projection lens. Such a method is described in U.S. Pat. No. 4,940,881. Placing a spinning element between the mask and the projection lens has the effect of moving the image in a circular orbit. This motion changes the ablated feature profile by moving the incident light at the surface of the substrate. The disadvantage of the method of U.S. Pat. No. 4,940,881 is that the radius of the orbit cannot be easily changed during the machining cycle. If the optical elements are wedge-shaped, as described in U.S. Pat. No. 4,118,109, the method also has the disadvantage that the angle of the beam is altered during the orbit, which limits the smallest possible beam displacement and complicates process control. An additional limitation is that hole wall slope profiles are limited to concave geometry (see FIG. 6), when used in conjunction with a conventional laser mask (e.g. one with simple apertures in the reflecting or absorbing coating for each ablated feature), except at very low fluences.
An apparatus and method for controlling an ablated orifice shape using two rotating optical elements is described in co-pending U.S. patent application Ser. No. 09/197,127, now U.S. Pat. No. 6,120,976, entitled "LASER ABLATED FEATURE FORMATION DEVICE" filed on like date herewith, and incorporated by reference herein. The invention of copending U.S. patent application Ser. No. 09/197,127 has the advantage over U.S. Pat. No. 4,940,881 in that the profile of the hole wall can be altered by controlling the relative rotational velocities and phase angle between the two rotating optical elements. In this manner, any desired hole profile (i.e., concave, convex or straight) can be obtained without requiring a complicated mask structure.
Yet another method for moving the image on the substrate utilizes a movable mirror between the mask and the projection lens. The mirror can be tilted in such a manner that the image moves in a prescribed orbit, thereby moving the incident light at the substrate. A major disadvantage of this method is the limited sensitivity of control, since a small tilt of the mirror can be a rather large displacement of the apparent mask position. Further, such mirrors must be of a minimum thickness to insure sufficient mechanical stability and flatness of the reflecting surface. This in turn, makes for a rather large inertia, and limits the bandwidth or highest speed of the device. When the system bandwidth is limited, it places limits on the scan patterns that can be effectively used to shape the holes.
An alternative to optically or mechanically moving the mask image is to actually move the substrate. This has a disadvantage, however, that the motion of the substrate must be very precise. The requirement for high precision is due to the fact that the projection lens of the ablation system shrinks the projection mask image down to the substrate to concentrate the laser energy. Consequently, the tolerances on the motion profile also shrink proportionately. This approach usually has the same inertial problems as the tilting mirror approach discussed above, except that the problem is further aggravated by additional mass of the substrate holders and motion stages used in typical automated systems.
As can be seen, there are multiple ways by which the profile of a laser ablated feature may be controlled to some degree. However, it can also be seen that the currently available methods have limitations which restrict their usefulness. What is needed, therefore, and what is provided by the present invention, is an apparatus and method for controlling the profile of laser ablated features which is very flexible in allowing the creation of multiple types of orifice profiles, while at the same time providing accurate and repeatable results. In the present invention, the mask itself is continually moved according to a prescribed set of coordinates for each and every laser pulse. The detailed trajectory of this motion has a strong influence on the final ablated hole shape. The ability to change the hole geometry without any additional optical element is a powerful yet flexible process parameter. Moving the mask itself within a certain prescribed trajectory can change the geometry of the ablated feature in a desirable fashion, including convex, concave and straight-walled features.